1. Field of the Invention
The present invention relates generally to the providing of an indication of the position of a movable member. More particularly, the present invention is directed to apparatus for sensing and transmitting a signal indicative of the position of a ferromagnetic movable member relative to a fixed structure within which the member moves.
2. Description of Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to the monitoring of the position of control rods in a nuclear reactor. Vertically movable control rods are typically used in nuclear reactors to maintain positive core reactivity control by controlling the overall reactor power level and providing the principal means of quickly and safely shutting down the reactor.
As is well known, in the case of a nuclear reactor, portions of the structure are normally isolated and sealed off to prevent exposure of operating personnel to dangerous amounts of radiation. Accordingly, the positioning of elements such as control rods for a reactor must be performed from a remotely located, central control station. Complicating matters is the fact that the control rods themselves must be positioned within sealed housings which extend from the main body of the reactor structure. Obviously, the less communication from the exterior to the interior of the control rod housing the better, and thus, conventional position indicating apparatus are not suitable for control rod position monitoring.
Theoretically, a hermetically sealed control rod drive, including a drive motor, is to be preferred, and the industry has widely adopted such drives. However, with the hermetically sealed drive, there is nothing extending out of the control rod housing to provide an indication of where the rod is positioned. The drive motor coils are positioned externally of the control rod housing and communicate with the control rod drive through magnetic coupling.
The monitoring of control rod position in a nuclear reactor is further complicated by the fact that the control rods are typically submerged in a fluid, and during operation of the reactor, the temperature of this fluid and of the rod itself becomes quite high (e.g., in excess of 500.degree. F.). The design of position monitoring means is thus complicated by the facts that, if portions thereof are to be physically affixed or connected to the control rod as has been past practice, such portions of the position indicating apparatus must be capable of withstanding high temperatures and must be able to operate while submerged in a fluid such as water.
These restrictions initially confined control rod position monitoring equipment designers to working with mechanical or electromechanical components which were suitable for use within the control rod housing. This limiting of flexibility of design required undesirable communication between the interior and exterior of the control rod housing, as mentioned above. In addition, in the case of a failure in the monitoring apparatus, repair thereof required a lengthy shut down of the reactor.
In U.S. Pat. No. 3,656,074, which is assigned to the same assignee as the present invention, there is described a position indicating apparatus which enables the position of a movable member, such as a nuclear reactor control rod, to be monitored without the necessity of providing communication between the interior and the exterior of the housing in which the movable member is situated. The system described therein includes a magnet located inside the control rod housing on the top of the control rod extension shaft, and a series of magnetically sensitive reed switches on the outside of the control rod housing. By sensing which switches are open or closed, the position of the control rod extension shaft, and therefore, the control rod can be determined.
This use of a magnet and reed switches has proved to be a more accurate and repeatable method of measurement than other methods used by nuclear reactor vendors. However, the magnetic position transmitter as it presently exists is not readily adaptable to all nuclear plants because not all plants were built with a magnet installed on top of the control rod extension shaft. The installation of a magnet within the control rod housing of existing, operating nuclear plants is prohibited by cost and radiation considerations.